The invention relates to a spotlight for shooting films and videos with light-emitting diodes arranged on a light-emitting surface, and to a method for setting the color characteristics emitted by the spotlight.
Lighting spotlights with light-emitting diodes (LEDs) are known which are used for example as camera attachment light for film and video cameras. Since the LEDs used therefor have either the color temperature “daylight white” or “warm white”, a continuously variable or exact switching on or switching over from a warm-white to a daylight-white color temperature is not possible and the color rendering when shooting films and videos is unsatisfactory in both variants.
Typical film materials for shooting films such as “cinema color negative film”, are optimized for daylight with a color temperature of 5600 K or for incandescent lamp light with a color temperature of 3200 K and achieve excellent color rendering properties with these light sources for illuminating a set. If, when shooting films, other artificial light sources are used for illuminating a set, then these must be adapted to the optimum color temperature of 3200 K or 5600 K, on the one hand, and have a very good color rendering quality, on the other hand. In general, the best color rendering level with a color rendering index of CRI≧90 . . . 100 is required therefor.
As when use is made of fluorescent lamps for illumination when shooting films or videos, however, it can happen in the case of artificial light sources having a non-continuous spectral profile that although said light sources achieve the required values for color temperature and color rendering, when used for shooting films they nevertheless have a considerable color cast by comparison with light from incandescent lamps or HMI lamps or daylight. In this case, this is referred to as an inadequate mixed light capability. This effect can also occur when use is made of different-colored LEDs in an LED spotlight. Thus, in a test with an LED combination optimized for a color temperature of 5600 K and a color rendering index of CRI=96, when shooting films, a considerable red cast was ascertained in comparison with HMI lamps. Experiments with daylight-white LEDs also did not yield satisfactory results with regard to the mixed light capability.
DE 102 33 050 A1 discloses an LED-based light source for generating white light which makes use of the principle of three-color mixing. The three primary colors red green blue (RGB) are mixed in order to generate the white light, in which case at least one blue-light-emitting LED, which is referred to as transmission LED and emits directly used light primarily in the wavelength range of from 470 to 490 nm, and also another LED, which operates with conversion and is correspondingly referred to as conversion LED and emits light primarily in the wavelength range of at most 465 nm, are combined in a housing. Disposed in front of both LEDs or a surface (array) constructed from a multiplicity of both types of LEDs is a common conversion surface composed of a potting or a glass plate with one or more luminescent materials, such that the luminescent materials completely convert the light from the conversion LED but allow the light from the transmission LED to pass through unimpeded.
Optimum color rendering for shooting films and videos cannot be ensured with this light source either, since there is in particular the risk of overemphasis of suppression of color components and thus corruption of the colors of an object illuminated by the light source. For this reason, a light source of this type is used predominantly in the entertainment sector.
Moreover, the luminescent material in the known light source is excited by short-wave radiation of max. 465 nm, whereby disadvantages with regard to efficiency and lifetime of the luminescent LEDs are to be expected.
US 2004/0105261 A1 discloses a method and a device for emitting and modulating light with a predetermined light spectrum. The known lighting device has a plurality of groups of light-emitting devices, each group of which emits a predetermined light spectrum, and a control device controls the power supply to the individual light-emitting devices in such a way that the radiation that results overall has the predetermined light spectrum. In this case, through a combination of daylight-white and warm-white LEDs and changing the intensities, it is possible to set any color temperatures between the warm-white and daylight-white LEDs.
Disadvantages of these methods include the likewise non-optimum color rendering when shooting films and videos and the lack of an opportunity to set a predetermined color temperature and an exact color locus. Depending on the choice of individual LEDs or groups of LEDs and the color temperature respectively set, it is necessary here to reckon with in part considerable color deviations from the Planckian locus, which color deviations can only be corrected by placing correction filters in front. What is more, the luminous efficiency is not optimal in the case of a warm-white setting of the combination of daylight-white and warm-white LEDs, since relatively high conversion losses occur in this case as a result of the secondary emission of the luminescent material. A further disadvantage of this method is that, for setting a warm- or daylight-white color temperature, a large proportion of the LEDs of the respective other color temperature cannot be utilized or can only be utilized in greatly dimmed fashion and, consequently, the degree of utilization for the color temperatures around 3200 K or 5600 K that are typically required when shooting films is only approximately 50%.